Update of Age and sex specific Natural mortality of the blue shark (Prionace glauca) in the North Pacific Ocean

Published as ISC/16/SHARKWG-1/06

In the last stock assessment of blue shark (Prionace glauca) in the North Pacific, age
and sex-specific natural mortality (indicated as “M” hereafter) of this stock was used as the
input data for Stock Synthesis model (Rice et al. 2014). In Rice and Semba (2014), the
estimator by Peterson and Wroblewski (1984) and Chen and Watanabe (1986) and the growth
equation by Nakano (1994) and Hsu et al. (2011) were used. The estimator of M applied is
divided into weight-based (Peterson and Wroblewski 1984) and age-based (Chen and Watanabe
1986). The age and sex-specific M based on the former estimator was adopted in the stock
assessment with the assumption of maximum age of 30. However, in the calculation by Rice
and Semba (2014), the coefficient assigned for the dry weight (1.92) was mistakenly applied to
wet weight of North Pacific blue shark, instead of that for the wet weight (1.28), which needs to
be corrected in the upcoming stock assessment.
While the weight-based estimator has been widely applied in the stock assessment of
fishery resources, the effect of variance of weight among individual is suggested to be large
compared to that of body length. For example, large difference in weight would be expected
between pregnant female and adult male with same age. In this context, we show the age and
sex-specific M based on the length-based method used in the stock assessment of Atlantic
yellowfin tuna (Thunnus albacares) this year (Method 2 in Walter et al. 2016) as well as the
modified M by Peterson and Wroblewski (1984). In addition, we calculated the age and sexspecific
M based on another estimators included in the review by Kenchington (2014) for
comparison. Regarding growth curve necessary for the estimation of M, von Bertalanffy growth
curve (VBGC) by Nakano (1994) and Hsu et al. (2011) are used for the sake of consistency with
the past estimates. In addition, the estimates based on VBGC by Fujinami et al. (2016) is also
indicated for reference.